THE PENNSYLVANIAN PERIOD. 



565 



was land 01 swamp vegetation. Had it been washed down from the 

 land where it grew into the situations where the coal is, and appar- 

 ently there is no other way in which it could have been drifted together, 

 it should have been mixed with earthy sediment, and the product, 

 even where the vegetable matter had undergone the necessary changes, 

 would have been very unlike the purer coal-beds now known. Fur- 

 thermore, the nearly uniform thickness of many of the coal-beds over 

 great areas, sometimes many thousand square miles, constitutes a 

 strong objection to the hypothesis that it was drifted together by 

 any process whatsoever. 



In support of the theory that the vegetation grew where the coal- 

 beds now are, many facts, in addition to the purity and the uniformity 



Fig. 252. — Showing a stump standing as it grew in Coal Measures near Glasgow, 



Scotland. 



of thickness already cited, may be noted. Thus (1) beneath each 

 coal-bed there is, as a rule, a layer of clay often filled with roots in 

 the position of growth. The clay seems to have been the soil in which 

 the coal vegetation was rooted in the earliest stages of the accumu- 

 lation. (2) In association with the coal-beds, the stumps of trees 

 are sometimes found still standing as they grew (Fig. 252). (3) In 

 the coal-beds, or in the associated layers of shale, imprints of the fronds 

 of ferns are found. They are often so numerous and their forms so per- 

 fect as to indicate that they were buried where they fell, rather than 



